Tunable acousto-optic method and apparatus

ABSTRACT

Polarized light waves propagate collinearly with acoustical waves in an optically anisotropic media. For nearly matched momentum vectors of the ordinary optical wave, extraordinary optical wave, and acoustic wave, diffraction of light into the orthogonal polarization occurs in a bandwidth determined by the birefringence and length of the media. The diffracted light may be selected by an output analyzer to form a band-pass filter. The operating point of the filter is varied by varying the frequency of the applied acoustical waves. A theory of operation is set forth together with example embodiments.

0R ansmtzgs United State Harris [54] TUNABLE ACOUSTO-OPTIC METHOD ANDAPPARATUS [72] Inventor: Stephen E. Harris, Palo Alto, Calif.

[73] Assignee: Chromatix, Inc.

[22] Filed: Nov. 13, 1970 [2!] Appl. No.: 89,379

Related [1.8. Application Data [63] Continuation of Ser. No. 820,873,May l, 1969, abandoned.

[52] US. Cl ..350/l49, 250/199, 350/l47, 350/l57, 350/161 [51] Int. Cl...G02i 1/24 [58] FieldoiSearch ..350/l47, I49, 150, l57, 160-16]; 250/I99 [56] References Cited UNITED STATES PATENTS 3,439,974 4/1969 Henryet al. ..350/l.57

OTHER PUBLICATIONS Dixon, Acoustic Diffraction of Light in AnisotropicMedia" 1 July 25, 1972 .l. Quant. Elect. Vol. QE-3, No. 2 (Feb. 1967)pp. 85- 93. Lean et al., Efl'icient Microwave Shear-Wave Generation byMode Conversion" Appl. Phys. Lett. Vol. 9, No. 10 (Nov. I5, 1966) pp.372- 374.

Dixon et al., New Technique for Measuring Photoelastic Tensors andApplication to Lithium Niobate Appl. Phys. Lett. Vol. 8. No. 8 (Apr. 15,I966) pp. 205- 207.

Harris et al., Acousto-Optic Tunable Filter" J.O.S.A. Vol. 59, No. 6(June I969) pp. 744- 747.

Primary E.\'mninerDavid Schon berg Assislun! Examiner-Paul R. MillerA!rorne \'-Flehr, Hohbaich, Test, Albritton & Herbert [57] ABSTRACTPolarized light waves propagate collinearly with acoustical waves in anoptically anisotropic media. For nearly matched momentum vectors of theordinary optical wave, extraordinary optical wave, and acoustic wave,diffraction of light into the orthogonal polarization occurs in abandwidth determined by the birefringence and length of the media. Thediffracted light may be selected by an output analyzer to form aband-pass filter. The operating point of the filter is varied by varyingthe frequency of the applied acoustical waves. A theory of operation isset forth together with example embodiments.

54 Claims, 8 Drawing Figures PITENTEBJum m2 3.679.288

saw 1 or 2 0.1 O l l l WAVELENGTH (A) 4| L49 44 47 f i f -a- -c- I LiNbO46 45 t INVENTOR. Y X STEPHEN E. HARRIS F/ G 5 1" 7%, W, m,

' 'ArmRNFYS ELECTRICAL POWER (mw) PATENTEOMZS SHEET 2 0F 2 3579288TRANSMITTED LIGHT 84 I I; If INPUT LIGHT N x Li 3 CdS THIN FILMPOLAR'ZER TRANSDUCER a. -6

E6500 F/G. 7 5 J L11 2 e000 .1 q A 2 E E 3 0 I5 5500 l I I I I 2; 750800 850 900 950 IOOOIOSO z ACOUSTIC FREQUENCY (MHz) 0 50- ik ,v" PULSED2 I 40- E 1 30 I w LU 0. CW F/G. a w 8 0: IO- INVENTOR. c STEPHEN E.HARRIS ATTORNEYS TUNABLE ACOUSTO-OPTIC METHOD AND APPARATUSCROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation of copending application Ser. No. 820,873, filed May 1,1969, entitled Tunable Optical Filter", now abandoned, and assigned tothe same assignee as the present invention, such parent application nowabandoned in favor of the present application.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 3 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION The present invention relates to opticalfilters and more particularly to a new type of electronically tunableacousto-optic device which utilizes collinear acousto-optic diffractionin an optically anisotropic media.

Heretofore, generally available optical filters have been limited intheir ability to rapidly change their frequency of operation. Tunablemechanical optical devices using interference films or layers are knownbut require the physical movement of mechanical elements as by revolvinga wheellike device or the like. Many applications such as in thebandpass filters used with lasers, which may themselves be tunable,require that such filters be readily tunable and simultaneously have asuitably narrow bandwidth. Particularly there is a need for an opticalfilter which is tunable in response to varying the frequency of aneasily controlled electric signal.

In general, it is an object of the present invention to provide atunable optical filter which will overcome .the above mentionedlimitations and disadvantages and for which the pass band may be tunedin response to varying the frequency of an applied electrical signal.

It is known that an acoustic wave travelling in a solid or liquidinduces a strain which can change the refractive index of the media anddiffracta light beam which is incident on the media. ln isotropic mediathe diffracted light is generally unchanged in polarization and thediffraction isfound to be particularly strong when the light is incidentat the Bragg angle. Generally, when an acoustical wave interacts with anoptical wave travelling at an angle to each other in an isotropic media,the acoustic wave perturbs the refractive index of the material and as aresult a certain fraction of the optical energy is scattered off in aparticular direction. Most acousto-optic reactions make use of the Braggangle for which scattering ef feet is most enhanced.

Heretofore, an experiment was conducted and reported by R. W. Dixon inThe lEEE Journal of Quantum Electronics, Volume QE-3, No. 2, of February1967, at pages 85-93, wherein an extremely narrow band, i.e., less thanone-tenth A, coherent light beam from a laser was collinearly diffractedon an acoustic wave within optically anisotropic birefringent media,namely, a quartz and alumina crystals. It was discovered and reportedthat when the frequency of the acoustic wave was at a certain frequencyrelated to the optical frequency of the light to be diffracted that aphase-matched cumulative collinear diffraction occurred which caused thelaser beam, which was polarized in a first linear direction, to becollinearly diffracted from the first polarization into a secondpolarization which was orthogonal to the first polarization.

The experimental apparatus consisted of a laser with an extremely narrowband coherent output beam, less than onetenth A bandwidth, which wasdirected through a polarizing prism into a birefringent crystal. Thelight beam was directed collinear with an acoustic wave. The acousticwave was launched into the crystal from an end thereof remote from thelaser via the intermediary of a fused quartz buffer rod having a cadmiumsulphide acoustic transducer affixed to the remote end thereof. Anoptically reflective coating was provided at the interface of the bufferrod and the birefringent crystal. The

linearly polarized incident light was collinearly diffracted on theacoustic wave into a polarization orthogonal to the incidentpolarization. The collinearly diffracted light was reflected from theremote end of the crystal and emerged from the input face of the crystalas an output diffracted beam. The diffracted beam was polarizationanalyzed by the polarizing prism to separate the light of the secondpolarization from the incident light of the first polarization.

A reduction in the cumulative diffraction was observed for departure Affrom the acoustic frequency which gave maximum cumulative collineardiffraction. It was speculated that the device could be employed as apolarization switch. However, it was concluded that this was probablynot a practical application because other available devices couldperform the same function more rapidly. lt was further speculated thatthe polarization switch could be employed inside a laser cavity as apolarization switch or as a loss modulator for producing short lightpulses and that when such use was untenable the light beam might beallowed to pass straight through the switch. However, no suchspeculative uses ,or devices were shown.

The subject prior art article also describes the mechanism of thephase-matched collinear diffraction in optically anisotropic media as aspecial case of Bragg diffraction in which the diffracted light isdifl'racted into a polarization orthogonal to the polarization of theinput beam.

SUMMARY OF THE INVENTION AND OBJECTS In the present invention it hasbeen discovered that the optical passband for collinear acousto-opticdiffraction in an optically anisotropic medium is a function of theinteraction length over which the optical beam is phase-matched with theacoustic wave, and is narrow band, on the order of several angstroms fortypical birefringent crystals of a length comparable to the length ofmedia previously employed by Dixon or Bragg diffraction, i.e., 5 cm.This narrow instantaneous passband characteristic was not recognized inthe prior art and is found to be readily tunable over wide opticalbandwidths by merely tuning the frequency of the acoustic wave over acomparable relative bandwidth. Moreover, it has been discovered that thelight to be collinearly diffracted does not have to be coherent but maycomprise a white light or other sources of coherent or incoherent lighthaving a wide bandwidth. In addition, a geometry for the acousto-opticdevice is provided which allows straight through transmission of thediffracted light beam and which has a maximum transmission of light,within the narrow passband, of nearly 100 percent as contrasted with amaximum transmission of less than percent for the prior art geometrywherein the light beam is internally reflected from a coating at theinterface of the acoustic transducer with the crystal.

As used herein, passband," narrow band or instantaneous passband" meansthe optical bandwidth of frequencies in which there is cumulativecollinear acousto-optic diffraction in the medium from the first to thesecond polarization with an acoustic wave of one frequency. -Tunableband" means the range of optical frequencies over which theinstantaneous passband of the filter can be tuned; broadband meansbroader band than the instantaneous passband and includes not only thetotal bandwidth of the incident light but the tunable band offrequencies of such light, as well. The passband" may cover a band offrequencies a few times less to several orders of magnitude less thanthe frequencies within the broad band of frequencies. I

The present invention uses a particular condition disclosed by Dixon fora strong interaction between the acoustic wave and the light wave, thiscondition being that the sum of the momentum vectors of the incidentlight, k and the acoustic wave, k, equals the mementum vector, E, of thediffracted light waves. As will become apparent, the choice ofsubscripts is peculiar to the example which will be set forth and standsfor extraordinary, acoustic and ordinary, respectively.

In the present optical filter, a birefringent crystal is used as themedium and is chosen to have an orientation such that an incidentoptical signal of one polarization will be diffracted into an orthogonalpolarization by collinear propagation through the crystal with theacoustic beam. The condition k I? IT, is also satisfied; where thesubscripts 3, a, and 0 stand for the extraordinary, acoustic andordinary waves respectively. For a given acoustic frequency, only a verysmall range of optical frequencies are found to satisfy the abovecondition of match of the momentum vectors. This small range offrequencies are found to be cumulatively difiracted into the orthogonalpolarization as the light beam and acoustic wave travel through thecrystal. The effect is noncumulative for op tical frequencies outsidethe narrow range, and, accordingly, incident light of other frequenciesis nearly unaffected.

The filter of the present invention includes an input polarizer todefine the polarization of the input beam, an output analyzer to selectlight having a polarization solely orthogonal to the polarization of theinput beam. It also includes an electronic source for generatingsuitable electric signal and an electric-acoustical transducer forconverting the electric signal into acoustical waves of the samefrequency. The transducer is mechanically coupled to the crystal forgenerating acoustical waves which propagate collinearly with thedirection of propagation of light waves in the crystal.

In one feature of the presentinvention, a beam of light which may have arelatively wide instantaneous or sequential optical bandwidth iscumulatively collinearly diffracted on an acoustic wave within anoptically anisotropic medium to cumulatively diffract light of a firstpolarization and of a relatively narrow optical bandwidth into light ofa second polariza tron.

In another feature of the present invention, a light beam to bediffracted enters an optically anisotropic medium on one side thereofand passes through the medium over an optical path which is generallycollinear with the path of an acoustic wave to cumulatively diffractlight from the first polarization into a second polarization over anoptical passband, such diffracted light emerging from the side of themedium remote from the side the light beam entered the medium, wherebythe maximum optical conversion to the second polarization within thepassband of the device is increased to nearly 100 percent.

Another feature of the present invention is the same as any one or moreof the preceding features including the additional feature of analyzingthe cumulatively diffracted light beam to separate light of one of saidpolarizations from light of the other of said polarizations.

Another feature of the present invention is the same as any one or moreof the preceding features including the additional feature of changingthe frequency of the acoustic wave to obtain a related change in theoptical frequency of the light which is cumulatively diffracted from thefirst polarization to the second polarization.

Another feature of the present invention is the provision of an acoustictransducer coupled to the optically anisotropic medium for launching anacoustic wave in such a direction as to be deflected at a boundary ofthe anisotropic medium from a path which is inclined to the optical pathinto a path which is generally collinear with the path of the light beamto be diffracted within the anisotropic medium.

These and other objects and features of the invention will becomeapparent from the following descriptions when taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS 1 of the filter of the presentinvention (not to scale).

FIG. 4 is a graph of b and 21r( n, 7],) versus wavelength for the filterof the present invention, where b is the birefringence corrected fordispersion and 2rr(n n,) represents the birefringence withoutdispersion.

FIG. 5 is a schematic representation of another embodiment of theinvention.

FIG. 6 is a schematic representation of one experimental optical filterconstruction in accordance with the present inventron.

FIG. 7 is a graph showing the tuning curve of the filter of FIG. 6.

FIG. 8 is a graph showing corrected peak transmission versus electricalpower input for the filter of FIG. 6.

Referring to FIG. I, a tunable optical filter of the present inventionis shown diagrammatically and includes an input polarizer 10 which ispositioned between a light source II and a medium 12 which, by way ofexample, is a birefringent crystal. The light source may be of any typesuch as a narrow band coherent source such as a laser or a broad bandlight source such as a glow discharge in a gas, or an incandescent lamp.The broad band light source may be incoherent white light or a coherentsource sequentially tuned over a broad band. The light need not beconfined to the visible spectrum. The input polarizer serves to passonly that light from the source which is polarized in a givenorientation to provide a polarized input light beam I3. The light beam13 enters one face of the crystal in which it propagates generally alonga predetermined axis and passes out the opposite face as an output beam14. Output beam 14 is then passed through a second polarizer or analyzer15 which has its axis of transmission orthogonally disposed with respectto the orientation of transmission of polarizer l0.

An acoustical transducer 16 is mounted in intimate contact with thecrystal and is connected to a suitable signal generator or source 17 oftime varying electric signals, the frequency of which can be varied. Thesource of electric signals, transducer and light source, as well aspolarizers l0 and 15, are of such type as may be generally available andneed not be further described herein, except as to frequency ofoperation and power requirements which will be hereinafter discussed.

The crystal I2 can, by way of example, be made of LiNbO provided withend faces 21, 22 cut at the Brewster angle to minimize reflections. Forthis crystal configuration the acoustic wave is brought in as alongitudinal wave 23 which is internally reflected and converted to ashear wave 24 upon reflection from input face 21. After reflection theacoustic shear wave 24 and the input light beam 13 propagate collinearlydown the Y axis of the crystal. For a particular combination of lightwave and acoustic wave frequencies, there is found a strong interactionbetween the light and acoustic waves in which the acoustic wavesdifiract the light waves from the polarization orientation of the inputbeam into the orthogonal polarization, as a result of the k vectormatching condition. Only a narrow frequency band of light is diffractedinto the orthogonal polarization and thus separated from the output beamby the output analyzer 15. This narrow passband is a function of theapplied acoustical frequency and can therefore be varied by varying thefrequency of excitation supplied by the signal generator 17.

This diffraction occurs as a cumulative effect for a very narrow band ofoptical frequencies, and is noncumulative by incrementalself-cancellation for other frequencies. The cumulative diffractioneffect occurs when the momentumvectors of the incident optical andacoustic waves satisfy the relation that their sum equals that of theoutput light beam. This condition is called phase matching. The narrow.band of frequencies satisfying this relation and diffracted into theorthogonal polarization is then passed by the output analyzer; while thelight of an optical frequency outside the narrow passband of the filterand thus of the initial polarization is blocked.

In the present filter, diffraction of light of one polarization intoanother by the acoustic wave will be particularly strong at a particularoptical frequency for a particular acoustic frequency. For one acousticfrequency, the optical frequency that will be most strongly diffractedis determined by the amount of birefringence, An, of the crystal. Thephenomena that makes it particularly strong is termed -phase matchingandoccurs when the generated polarization wave travels at the same velocityas the free electromagnetic wave (a light wave propagating in thecrystal in the absence of the acoustic wave).

When light of one polarization passes through the crystal, the acousticwave mixes with it to generate what can be termed the travellingpolarization wave which moves according to its momentum vector, which isthe sum of the momentum vectors of the incident light and the acousticwave. Now, if the birefringence of the crystal is chosen such that thatpolarization is moving at the same velocity that the freeelectromagnetic light wave travels, the diffraction process will beparticularly strong.

On a macroscopic basis, a crystal is chosen with a certainbirefringence, and by changing the acoustic frequency put in, theoptical frequency which will most effectively be difi'racted into theorthogonal polarization is also changed. So, broadly stated, the filterincludes an input polarizer to establish polarization of the inputlight, a birefringent crystal with an acoustic wave in it, and an outputpolarizer turned perpendicular to the input polarizer so only thatfrequency which is particularly well diffracted will be passed by theoutput polarizer. And, when the acoustic frequency is changed, thebandpass of the filter is changed. The following is a detailedexplanation of the theory of operation of the filter of the presentinvention.

The following is an explanation of the operation of the filter as itwould apply for the case of LiNbO Assume the input light beam to be anextraordinary wave polarized along the optic axis, Z, of the LiNbO,crystal. The output or diffracted optical beam will then be an ordinarywave polarized along the x axis of the crystal. The acoustic wave whichis necessary to accomplish the diffraction into the orthogonalpolarization along x is an S shear wave generated at surface 23 of thecrystal. The three waves are then taken as plane waves and aremathematically represented as:

The quantities 0),, m m and k,., k k are the angular frequencies and I?vectors of the input optical wave, output op tical wave, and acousticwave, respectively. The symbol A denotes variables which have thecomplete time and spacial dependence, as opposed to the envelopevariables E,( y), etc. The acoustic wave mixes with the input opticalsignal to produce forcing optical polarization waves at frequencies a),a) and w, (0,. These forcing waves propagate with k vectors of magnitudek, k,, and It, k,,, respectively. Only if the k vector of this forcingwave is equal or nearly equal to that of the freely propagatingelectromagnetic light wave, will a cumulative interaction overmanywavelengths take place. In

traordinary index, which for forward propagating waves requires phasematching such that k, I? k this, in turn, results in the frequency ofthe ordinary wave (the output frequency) being greater than that of theextraordinary wave (input frequency) by (0 The interaction between theacoustic and optical waves takes place as a result of the photoelasticeffect. This effect is described as a perturbation of the elements ofthe impermeability tensor b such that Ab p S where Put-1 are thecomponents of the photoelastic tensor and S is the propagating strainwave. This perturbation of the impermeability tensor is and make use thefacithat iirs'iatv'iy vary: ing functions of y, then the followingcoupled complex equations are obtained Ty SJ.) exp ([Aky) as 2 we (4)where there is defined a k vector mismatch Ak= k k, k,,. In theseequations, the acoustic wave is assumed to propagate losslessly and thusthe acoustic strain S is assumed to be independent of position in thecrystal.

Equations (4) are now solved subject to the boundary condition that E, 0and E,= E,(0) at y 0. The ratio of the output power at y L, PAL), to theinput power at y 0, P,(0) is It is noted that the frequency of thetransmitted optical signal difiers from that of the portion of the inputsignal with which it interacts by the acoustic frequency to There isalso an insignificant Manley-Rowe type power gain of magnitude (n /wwhich will be neglected in the following discussion.

The following is a discussion of transmittance, tuning rate, bandwidth,and aperture for the filter. From E0. (5) it is is seen that the maximumtransmittance of the filter will be attained when the input opticalfrequency is such that the momentum mismatch Ak 0. For this condition,

"0 31 41 l 2 l 2 pWA h N H (7) where M is the optical wavelength, p isthe density of the medium, V is the acoustic velocity, P is the totalacoustic power, and A is the area of the acoustic and optical beams.

For a S-cm-long crystal of LiNb0 at a central transmission I LiNbO theordinary refractive index is greater than the exfrequency of =500() A; p0.155, n 2.3, n, 2.2, p

Let

where Ay is the change in wave numbers of the optical frequency from thecenter frequency of the filter. From Eq. (5 theoptical-frequency-response function of the filter HQ) may then bewritten Ak=( a) FIG. 2 shows the transmittance HQ) plotted versus thenormalized frequency variable bLAy. The half-peak transmittances of theprimary lobe of the filter occur when bLAy a i 2.5. For LiNbO theconstant b may be obtained by differentiation of the Sellmeier equationsof Hobden and Warner. (See M. V. Hobden and J. Warner, Phys. Rev.Letters 22, 243 (1966)). The result of this differentiation is given inFlG. 4, as a function of the optical wavelength, at a temperature of200C. This temperature was chosen because LiNb0, exhibits optical damageat temperatures lower than about 160C. FIG. 4 shows that b is somewhatlarger than the value 2-rr(n n,) that it would have in the absence ofoptical dispersion. The quantity Zn-(n n,.) is also shown, in FIG. 4.For a S-cm-Iong crystal of LiNb at 5,000 A, the total half-peakbandwidth of the first lobe of the filter S/bL) is about l.25 em 0.31 A.

Electronic tuning of the filter is accomplished by changing thefrequency of the output of the electrical signal source 17 and theassociated acoustic wave, thereby changing the length of its k vector.The acoustic frequency that will yield peak transmittance at an opticalwavelength h is fil 0)( 0 "P) where V is the acoustic velocity. ForLiNb0 V 4 X l0 cm/sec and n n, may be obtained from FIG. 4. Thenecessary acoustic frequency for peak transmittance at 5,000 A is 680Me, and the region from 7,000 to 4,000 A can be tuned by changing theelectrical and acoustic frequency from about 428 Me to'about 990 Me. Therate of change of optical wave number per cycle change of the acousticfrequency is AyIAf,,= 21r/h VA, where b is defined in Eq. (8) is about20 wave numbers per Mc change of the acoustic frequency. As the acousticfrequency is changed, the acoustic power should be varied inversely asthe square of the acoustic frequency, if 100 percent peak filtertransmittanceis to be maintained [note eqs. (6), (7), and 10)].

The optical angular aperture of the filter at the input frequency corresponding to peak collinear transmittance is determined by k vectormismatch. The half-peak, half-angle aperture occurs when ML 5 A. Fornearly collinear propagation, we obtained from FIG. 4

2 JALAINIJ'Z The half-angle aperture taken inside the crystal is thenabout :11 (k/LAn). This is magnified by refraction at the input of thecrystal to yield a total aperture of about 2n, (h/LAn). For a 5 cmcrystal of LiNbl); at h 5,000 A, this yields a half-peak apertureexternal to the crystal of approximately 0.02 radians or l.l5.

One way to view the operation of the filter of the present invention isto say that the acoustic wave travels through the crystal, on amicroscopic basis, and distorts the crystal symmetry and thereby changesthe birefringence in such a manner that the polarization of an incidentlinearly polarized light wave is changed or rotated to a polarizationorthogonal to the incident beam. The k matching condition insures thatthis process will take place cumulatively over the full length of thecrystal. To the extent that k matching is not there, it only takes placein an infinitesimal increment which is then cancelled by the nextinfinitesimal increment so that the process is not cumulative.

Two considerations are involved in selecting the orientation of thebirefringent crystal. One is to have at least a reasonably large,efiective photoelastic constant for diffraction from one polarizationinto the other. This has nothing to do with k matching. it is just aquestion of the size of the coefi'icient governing the diffractionprocess. It depends on the crystal tensor and the particular crystalclass. For a given crystal, like lithium niobate, there are certaincrystal orientations for which this photoelastic constant is larger thanother crystal orientation. The above analysis shows that propagationalong the Y axis in lithium niobate was a satisfactory choice of aneffective crystal orientation.

The second consideration in choosing the crystal orientation relates tothe adjustment of the birefringence. A given crystal class or givencrystal like lithium niobate has a certain maximum birefringence. Thismaximum birefringence occurs when you propagate at to the optic axis.For other directions of propagation in the crystal, the birefringence issmaller. So by picking an angle with respect to the optic axis, onecould select the amount of birefringence. One chooses a particularbirefringence for the acousto-optic filter because it is thebirefringence that determines the acoustic frequency which will phasematch with a given optical frequency. So, for

instance, suppose one wants to work in the visible region of thespectrum, tuning optical frequency from red to blue. One would then likethe acoustic frequency to lie in a reasonable range. One can then selectthis angle to adjust the birefringence to cause the acoustic frequencybe in a comfortable region. For lithium niobate, it is found that thewave can propagate at 90 to the optic axis, using the full birefringenceand this also turns out to be the angle of the largest photoelasticconstant. So in this case, all of the numbers are particu larlyconvenient in that the acoustic frequency lies in a convenient range andthe photoelastic constant is large enough to be of practical interest.The birefringence is inversely related to the bandwidth, so thatselection of the largest birefringence available will correspond to anarrow bandwidth, and, selection of smaller birefringence corresponds towider bandwidths. The largest birefringence also implies the largestacoustic frequency as spelled out by equation 10.

The following is a discussion of tuning rate and bandwidth of operation.The parameter b is a material parameter which combines birefringencewith dispersion, the change of birefringence with wavelength. The tuningrate in units of optical wavenumbers per cycle change in acousticfrequency is given by 21r/bV where b is defined in equation (8) andplotted in FIG. 4. It turns out that b is not much different than 211times the birefringence and that is why they are both plotted, so thatone does not have to know his material in that detail to get anapproximation of the value of b by estimation of 21r( n,

The bandwidth of this device is inversely proportional to both thequantity b and the quantity L, where L is the light path length of thenonlinear crystal. Referring to the sin x/x curve of FIG. 2, one sees aplot of transmission versus bLAy where y is in wave numbers. So for aparticular crystal length, and a particular b, the bandwidth of thefilter is defined in wave numbers. So in particular, we note making thecrystal longer, narrows the range of Ay over which the crystal willtransmit. Then, as estimated from equation (9), a five centimetercrystal of lithium niobate has a total half peak power bandwidth of thefirst lobe of approximately 5/bL or about one-third of an Angstrom at awavelength of 5,000 A.

For the present example, the most limiting factor of the present filteris the difficulty of obtaining large apertures. Since 14 mw ofpropagating acoustic power per mm of crystal aperture is required, al-cm-square aperture needs an acoustic power of 1.4. watts. Broadbandr-f to acoustic transducers can now be constructed with about 10 dBconversion loss, thus requiring an r-f power of 14 watts.

The present analysis has neglected the acoustic attenuation that occursas the acoustic wave propagates down the crystal. At room temperature,this attenuation should be about 0.75 dB/cm at 1,000 Mc; and should varyapproximately as the square of the acoustic frequency. lts effect willbe equivalent to shortening the crystal and will thus lead to somewhatlarger bandwidths and necessitate somewhat higher acoustic drive powers.

LiNb and the filter configuration of FIG. 1 are only one of a number ofpossible crystals and configurations that could be employed. Theadvantage of this configuration is that it allows the acoustic wave tobe brought in at right angles to the light, and thus does not requirethe light to pass through an acoustic transducer. A disadvantage is thatfor shear-wave propagation down the y axis there is an approximately 7divergence between the direction of the acoustic power flow and theacoustic k vector. This requires that the filter aperture be at leastone part in 10 of the crystal length. However, other crystalorientations allow difiraction into the orthogonal polarization and donot exhibit this divergence. For example, collinear propagation of alongitudinal acoustic wave and the optical signal down the x axis of aLiNbO crystal accomplishes the desired result.

Two other materials that may be useful for this type of filter aresapphire and quartz, which have the same photoelastic tensor as doesLiNb0 The birefringence of both of these materials is about one-tenththat of LiNb0 As a result, the necessary acoustic frequencies would becentered about 70 Mc instead of 700 Me as in the LiNbO, filter. Both thetuning rate and also the bandwidth of these filters (for the samecrystal length) would be about ten times as large as that of the LiNb0filter. The angular aperture would be about three times as large as thatof a LiNb0, filter of the same length. As a result of the lowerrefractive indices of these crystals, about 10 to times as much acousticpower would probably be required to obtain the theoretical 100 percentpeak transmittance. However, this might be off-set by use of longercrystals.

Referring to FIG. 5, another embodiment of the invention is shown andconsists of a source of light 41 providing an output light beam which ispassed through a polarizing prism 42 as,

for example, of the Rochon type, which also serves as an output analyzerfor light of orthogonal polarization which is travelling in the oppositedirection. The polarized light output of prism 42 is then passed backand forth through a birefringent crystal 44, such as LiNb0 Crystal 44 isprovided with end faces 43 and 45 which are plane parallel, face 43serving as an input surface for admitting light received from prism 42.Face 45 is provided with a reflective coating so that light traversingthe crystal is reflected by the surface 45 and passed back toward thesource through surface 43.

An electro-acoustical transducer 46 is coupled to reflective surface 45and is connected to a suitable signal generator or source 47 of electricsignals as in the device of FIG. I which is tunable, for example, from750 to 1,050 MHz. The acoustic output of the transducer is coupled tothe crystal and sets up acoustic waves. These waves can couple to anacoustic resonance resulting from making the end faces plane parallel.By using the acoustic resonance of the crystal, the amount of acousticdriving power required for operation can be reduced.

passing back through the crystal. Such reflected waves will betransmitted by prism 42. The narrow band of wavelengths which are phasematched and rotated into the orthogonal polarization will be separatedout by the prism 42 and reflected to form an output beam 49.

An alternative configuration is to make the input surface of the crystalinclined at an angle to the direction of wave propagation between thefaces to thereby present an inside surface to reflect impinging acousticwaves off at an angle. In that case, an acoustic load would be coupledto the side of the crystal in a position to receive and absorb acousticwaves reflected from the inside of the input surface and the entirecrystal would be set at an angle to the input light beam so as tocompensate for refraction.

Referring to FIGS. 6-8, there is shown an experimental example of anoptical filter of the present invention in which tuning from 7,000 to5,500 A has been obtained by changing the acoustic frequency from 750 to1.050 MHz. A band pass of less than 2 A, a corrected peak transmissionof 50 percent, and an average skirt rejection ratio of about 45 dB havebeen obtained.

In the filter configuration described here, and shown in FIG. 6,linearly polarized light from an input output analyzer 80 is normallyincident on a 1.8-cm-long crystal 82 of 90 cut LiNb0;,, and propagatescollinearly with a longitudinal acoustic standing wave induced byacoustic transducer 84, along the x axis of the crystal. The incidentlight may be polarized along either the y or z axes. Diffraction intothe orthogonal polarization occurs via the p,, photoelastic constant,and is only cumulative if lk,,| [k,| |k,,| where the subscripts o, e,and a denote the ordinary and extraordinary optical waves, and theacoustic wave respectively. This will be the case if the optical andacoustic frequencies )1, and f,, are related by Y c l fl V mfl where c/Vis the ratio of the optical velocity in vacuum to the acoustic velocityin the media, and An is the birefringence of the crystal. For LiNbl),for the orientation shown, V E 6.57 X 10 and An 5 0.09; and thus)", 25.l( l0 )f,.

The pass band of this type of filter is of the form sin x/x and has ahalf-power width determined by the condition |AkL'| 2', where L' is thetotal (round-trip) interaction length of the optical and acousticfields. Neglecting the dispersion of the refractive indices, this givesa half-power bandwidth of about HIV 5 (13 where L is the length of theLiNbO crystal. This gives a theoretical bandwidth of about 3.1 cm orabout 1.1 A at 6,000 A. It should be noted that no secondary orhigher-order pass bands are present.

The LiNb0 crystal 82 was polished flat and parallel in order to enhancethe longitudinal acoustic resonances which occured every 0.l8 Mc/sec.From Eq. l2 this corresponds to an optical frequency spacing of about 3cm", which from Eq. 13 is about equal to the theoretical resolution ofthe filter. The width of the acoustic resonances-was about 0.03 Mc/sec.yielding an acoustic finesse of 6. The CdS thin film transducer had anarea of L8 mm and was centered at about 864 MHz (corresponding to 6,328A). It had a half-power tuned bandwidth of about 15 percent, and a l7-dBelectrical-to-acoustical conversion loss at 864 MHz. 7

The experimental tuning curve of the filter is shown in FIG. 7 and wasobtained using a mercury are light source and Beck reversionspectroscope. Further tuning was limited by the The operation of thedevice of FIG. 5 is similar to that of bandwidth of the CdS transducer.The resolution over this FIG. 1 except that the polarizing prism 42serves also as an analyzer. Thus, input electromagnetic waves arelinearly polarized (in a plane perpendicular to plane of drawing) by theprism before entering the crystal 44 and if undiffracted range wassufi'icient to just resolve the successive acoustic resonances (3 cmapart). The average rejection ratio against frequencies outside of thepass band was 45 dB, and is a measure of the extent to which the crystalis strain free. The halfwill remain so polarized after reflection mi ofsurface 45 and power angular aperture was estimated to be about0.03 rad.

Peak filter transmission was measured using a He-Ne laser, and resultsare shown in FIG. 8. The data shown are corrected for optical lossesexperienced at the crystal input face and at the acoustic transducer.The reflection coefficients of these surfaces were 15 and 58 percent,respectively. It is seen that at about 500 mW of electrical input powercorrected transmission has saturated at 50 percent. This maximum resultsbecause of additional conversion to sidebands which have the samepolarization as that of the input signal; and also possible as a resultof thermal effects. As a result of crystal heating, the maximumcorrected cw transmission was'limited to about 30 percent.

Thus, there has been provided a new and useful tunable optic filter ofnarrow, controllable bandwidth. A particular advantage of the filter isits direct response to an applied electric signal from a commonlyavailable signal generator. Such signal generators are capable of rapid,swept output to give the present filter instantaneous frequency scanningpotential as opposed to the relatively slow action of mechanicallyrotated filter wheels. in the future this filter can be used as tunableYIG filters are now used in microwave work, and as the laser becomesmore versatile and tunable, many applications of this filter areexpected. A further application lies in its use as a monochromator orspectrometer which its potentially small size and ease of constructionare of benefit.

.Further, as regards the acoustic transducer 23 or 46, many of thevarious types of suitable acoustic-optic crystal materials arepiezoelectric, such as quartz and lithium niobate, therefore, theacoustic waves can be excited directly in the crystal by electrodes,without the need of a separate acoustic transducer coupled to thebirefringent medium, 12 or 44.

As regards the light source ll or 41, it is desirable, for certainembodiments of the present invention, that the light source be broadbandrelative to the instantaneous bandwidth of the light shifted from onepolarization to an orthogonal polarization. Thus, in a sense, the termbroadband" as employed to characterize the light source means broaderthan the instantaneous optical passband of the acoustic-optic device.However, light sources are readily available providing octaves ofoptical bandwidth in the visible and invisible bands. .Thus, in thelatter sense a broadband light source can be considered as a lightsource having an optical bandwidth in excess of 10 percent.

As used herein, light is defined to mean electromagnetic radiation. Suchlight may not be confined to the visible spectrum. Likewise, optical"need not be confined to the visible spectrum.

The benefits of collinear diffraction are: (l) the stringent angulartolerance characteristic of Bragg diffraction is relaxed. (2) theinteraction volume of the acoustic and optical beams is enhanced becauseof the collinear propagation; therefore, the efficiency of lightconversion can be increased to nearly [00 percent and the acoustic powerrequired for efficient conversion drastically decreased.

in some birefringent crystals the acoustic phase velocity k vector andthe group velocity are not collinear. The angle between them may be asmuch as as in the case of quartz. In such a case, the light beam may becollinear with either the phase or the group velocity with correspondingadvantages and disadvantages.

if the light beam is collinear with the phase velocity, the advantage ofslow dependence of bandwidth center frequency upon divergence isretained with the disadvantage that the light beam soon walks out of theacoustic beam because the energy propagates in the direction of thegroup velocity and is not collinear with the light beam. Thus, nan-owband characteristics can be obtained in this case at the expense ofeither larger acoustic aperture or higher acoustic power.

On the other hand, if the light beam is made collinear with the groupvelocity, the advantage of efficient utilization of acoustic energy isretained but the angle between the acoustic k vector and the optical kvector is no longer zero and the shift in the center of the opticalpassband with divergence in the The bandwidth of the acoustic-opticfilter is inversely related to the length of the interaction path Lthrough the crystal. in resonant acoustic-optic filters the path lengthcan be relatively short and, thus, the bandwidth is relatively wide. Insuch cases the k vectors for the optical and acoustic waves can havesubstantial divergence while still retaining the benefits of collineardiffraction.

The collinearly propagating optical and acoustic waves have beendescribed as travelling in the same direction. However,

20 the filter will also operate if they travel in opposite directions.

Let us examine the case of LiNbtl where the ordinary optical index isgreater than the extraordinary optical index. Further assume that theacoustic and optical waves travel in the same direction. First, assumethat the incident optical wave is an ex- 2 5 t raordinary wave. In thiscase the F vector condition k, 75,,

k will be satisfied. In this case the light will also be up-shifted bythe acoustic frequency w Alternatively, consider the case where theincident optical wave is an ordinary wave; and again, the acoustic andoptical waves travel in the same direction. In this case the conditionF, I? F, will again be satisfied, but in this case the incident opticalsignal will be down-shifted by the acoustic frequency to Consider nextthe two remaining cases. First, let the optical signal be anextraordinary wave but let the acoustic wave travel in the oppositedirection to the optical wave. ln this case, the vector condition F,-F,, k will be satisfied and the acoustic wave will be down-shifted infrequency. Finally, consider the case where the incident optical wave isan ordinary wave and the acoustic wave travels in the opptgtitedirection to the optical wave. Now, the vector condition k k, k, willagain be satisfied, but this case the incident optical signal will beup-shifted in frequency.

Thus, in general, the necessary k vector matching condition will be ofthe form k :t k, k,,, where the 1 sign is determined by both therelative direction of optical propagation and acoustic propagation; andalso on whether the extraordinary index is greater than or less than theordinary index.

I claim:

1. In an acousto-optic apparatus for difiracting light of a firstpolarization and first bandwidth into light of a second polarizationincluding, optically birefringent medium means disposed to receive abeam of light having a first optical bandwidth and to be difiracted froma first polarization into a second polarization, means for exciting anacoustic wave in said optically birefringent medium at a frequencyrelated to the optical frequency of the light to be diffracted from thefirst to the second polarization, such acoustic wave having a path ofpropagation within said optically birefringent medium which issubstantially collinear with the optical beam path for cumulativelycollinearly diffracting the light beam on the acoustic wave within theoptically birefringent medium to selectively diffract light of the firstpolarization, and within a second instantaneous passband of opticalfrequencies narrower than said first bandwidth and related to thefrequency of the acoustic wave, into light of the second polarization,and means for developing said light beam to be difiracted with saidfirst optical bandwidth wider than said second narrower instantaneouspassband.

2. The apparatus of claim 1 including, means for analyzing thedifiracted light beam to separate light of one of said polarizationsfrom the light of the other of said polarizations.

3; The apparatus of claim 1 including, means for varying the frequencyof the acoustic wave within said optically birefrinoptical beam is amore rapid function. ln intermediate cases i gent medium over a band ofacoustic frequencies to tune the second instantaneous passband ofoptical frequencies over an optical bandwidth.

4. The apparatus of claim 1 including, means for linearly polarizing thelight beam to be difi'racted in the direction of the first polarization.

5. The apparatus of claim 1 wherein said optically birefringent mediummeans is disposed to receive the beam of light to be diffracted with alight beam path through said medium such that the light beam enters saidmedium at one side, traverses a beam path within said medium, andemerges from said medium as a diffracted light beam on a side of saidmedium opposite the side of light beam entry.

6. The apparatus of claim 1 wherein said means for exciting an acousticwave in said optically birefringent medium includes, acoustic transducermeans coupled to said optically birefringent medium means for directingan excited acoustic wave into the medium at such an angle as to bedeflected at a boundary of said optically birefringent medium from apath which is inclined to the optical beam path into a path which isgenerally collinear with the optical beam path within said opticallybirefringent medium.

7. The apparatus of claim 1 wherein said means for developing said lightbeam of first polarization and first bandwidth develops said light beamas a beam of incoherent light.

8. In a acousto-optic apparatus for difiracting light of a firstpolarization into light of a second polarization including, opticallybirefringent medium means disposed to receive a beam of light to bedifi'racted from a first polarization into a second polarization suchbeam of light having a beam path through said medium such that the lightbeam enters the medium at one side, traverses a beam path within themedium, and emerges from the medium as a diffracted light beam on a sideof the medium opposite to the side of light beam entry, means forexciting an acoustic wave in said optically birefringent medium at afrequency related to the optical frequency of the light to be diffractedfrom the first to the second polarization, such acoustic wave having apath of propagation within said optically birefringent medium which issubstantially collinear with a substantial component of the optical beampath for cumulatively collinearly diffracting the light beam on theacoustic wave within the optically birefringent medium to diffract lightof the first polarization and of an optical frequency related to thefrequency of the acoustic wave into light of the second polarization.

9. The apparatus of claim 8 including, means for analyzing thediffracted light beam to separate light of one of said polarizationsfrom light of the other of said polarizations.

10. The apparatus of claim 8 including means for varying the frequencyof the acoustic wave in said optically birefringent medium to vary theoptical frequency of the light diffracted from the first polarizationinto the second'polarization.

11. The apparatus of claim 8 including, means for developing said lightbeam to be diffracted with an optical bandwidth wider than theinstantaneous passband of the light which is diffracted from the firstpolarization to the second polarization.

12. The apparatus of claim 8 including, acoustic transducer means forexciting and directing the acoustic wave into said opticallybirefringent medium at such an angle as to be deflected at a boundary ofsaid optically birefringent medium from a path which is inclined to theoptical beam path into a path which is generally collinear with theoptical beam path within said optically birefringent medium.

13. The apparatus of claim 8 wherein said birefringent medium means is abirefringent crystal.

14. The apparatus of claim 29 including, means for linearly polarizingthe incident light beam in the direction of the first polarization.

15. In an acousto-optic apparatus, a light beam of a first polarizationand bandwidth which is to have only a portion of its total handwithdiffracted into light of a second polarization, optically birefringentmedium means disposed to receive the beam of light having a firstoptical bandwidth and to be diffracted from a first polarization into asecond polarization,

means for exciting an acoustic wave in said optically birefringentmedium at a frequency related to the optical frequency of that portionof the bandwidth of the light to be diffracted from the first to thesecond polarization, such acoustic wave having a path of propagationwithin said optically birefringent medium which is substantiallycollinear with a substantial component of the optical beam path withinsaid birefringent medium for cumulatively collinearly diffracting thelight beam on the acoustic wave within the optically anisotropic mediumto diffract light of the first polarization, and within a secondinstantaneous passband of optical frequencies narrower than said firstbandwidth of light and related to the frequency of the acoustic wave,into light of the second polarization, means for varying the frequencyof the acoustic wave within said optically birefringent medium over aband of acoustic frequencies having a bandwidth in excess of 10 percentto tune the second instantaneous passband of optical frequencies forlight diffracted from the first polarization to the second polarizationover a comparable optical bandwidth in excess of l0 percent.

16. The apparatus of claim 15 wherein said means for varying thefrequency of the acoustic wave includes an acoustic transducer meanshaving a bandwidth in excess of 10 percent for exciting acoustic wavesover a relatively wide band of acoustic frequencies.

17. The apparatus of claim 15 wherein said means for varying thefrequency of the acoustic wave over a band of acoustic frequenciesincludes, tunable radio frequency generator means for generating anelectrical signal tunable over a band of frequencies as wide as the bandof acoustic frequencies to be generated within said opticallybirefringent medium, and acoustic transducer means responsive to thetunable electric signal for exciting the acoustic waves to be excited insaid birefringent medium means.

18. The apparatus of claim 15 including, means for developing said lightbeam to be diffracted with said first optical bandwidth wider than saidsecond instantaneous passband of light which is diffracted from thefirst polarization to the second polarization.

19. The apparatus of claim 15 including, means for analyzing thediffracted light beam to separate light of one of said polarizationsfrom light of other of the said polarizations.

20. The apparatus of claim 15 wherein said light beam of firstpolarization is a beam of incoherent light.

21. An acousto-optic apparatus comprising, means for developing a firstlight beam having a first band of frequencies with a predeterminedpolarization, an anisotropic crystal disposed to receive said beam, saidcrystal being selected and arranged to propogate said light beam as aparticular wave in a birefringent system, said crystal being furthercapable of diffracting and propagating a second light beam with a secondband of frequencies within said first band and being of orthogonalpolarization to said first beam, as another wave of said birefringentsystem in response to acoustical waves in the medium, means coupled tosaid crystal for developing acoustical waves of predetermined frequencyin said anisotropic crystaLthe frequency of said acoustical wave beingselected to define a phase-matched relationship with said first lightbeam such that the vector sum of the momentum vectors of said secondband of frequencies narrower than and within said first band offrequencies of the incident light beam and the acoustical wave are equalto that of said second light beam orthogonally oriented to said inputlight beam, whereby said second narrower band of frequencies of saidincident light beam satisfying said phase-matched relationship for agiven acoustical wave frequency is diffracted into said orthogonalpolarization, and an output analyzer aligned to selectively pass thelight waves orthogonally related to the polarization of said input lightbeam.

22. An acousto-optic apparatus as in claim 21 in which said crystal isprovided with end faces forming an acoustic resonator, a reflectivesurface carried on one of said end faces for internally reflecting lightwaves from within said crystal.

23. An acousto-optic apparatus as in claim 21 in which said crystal hasa pair of end faces, one of said end faces being constructed andarranged to admit said first light beam into said crystal, lightreflective means being provided at said other end face for internallyreflecting light impinging thereon.

24. An acousto-optic apparatus as in claim 22 wherein said meansfordeveloping acoustical waves is coupled to said end face having saidlight reflective means.

25. The apparatus of claim 2i wherein said means for developing saidfirst light beam develops a first light beam as a beam of incoherentlight.

26. An acousto-optic apparatus including a first light beam having afirst band of frequencies and predetermined polarization, an anisotropiccrystal medium disposed to receive said first light beam, said crystalbeing selected and arranged to propagate said first light beam as aparticular wave in a birefringent system, said crystal being furthercapable of diffracting and propagating a second light beam having asecond band of frequencies within and narrower than said first band offrequencies at orthogonal polarization to said first beam as anotherwave of said birefringent system in response to acoustical waves, meanscoupled to said medium for developing acoustical waves of predeterminedfrequency in said anisotropic medium, the frequency of said acousticalwaves being selected to define a phase-matched relationship such thatthe vector sum of the momentum vectors of said second narrower band offrequencies of said incident light beam and the acoustical wave areequal to that of said second band of frequencies of said second lightbeam orthogonally oriented to said first input light beam, whereby saidsecond band of frequencies of said incident light beam satisfying saidphasematched relationship for the given acoustical wave frequencies arediffracted into said orthogonal polarization.

27. The apparatus of claim 26 wherein said first light beam is a beam ofincoherent light.

28. An optical filter for deriving a narrower band of frequencies from alight beam having a first band of frequencies comprising, means fordeveloping an input light beam having a first band of opticalfrequencies and having a predetermined first polarization, an opticallyanisotropic medium disposed to receive said light beam, said mediumbeing selected and arranged to propagate said light beam as particularlight waves in a birefringent system, said medium being further capableof diffracting and propogating a second band of optical frequenciesnarrower than and within said first band of optical frequencies, oforthogonal polarization to said first band as another wave of saidbirefringent system, in response to an acoustical wave in said medium,means coupled to said medium for developing acoustical waves ofpredetermined frequency in said anisotropic medium, the predeterminedfrequency of said acoustical waves being selected to define aphase-matched relationship such that the vector sum of certain momentumvectors of the incident light beam and the acoustical wave aresubstantially equal to the momentum vector of said second narrower bandof light waves, whereby said second narrower band of optical frequenciesof the incident light beam satisfying said momentum vector condition fora given acoustical wave frequency is diffracted into said orthogonalpolarization to provide said second narrower band of opticalfrequencies, an output analyzer aligned to analyze the light wavesorthogonally related to the first polarization of said input light beam,and means for varying the frequency of the acoustical waves to vary thefrequency of said second narrower band of the light waves beingdiffracted into said orthogonal polarization.

29. An optical filter as in claim 28 wherein said means for developingan acoustical wave includes an electro-acoustical transducer, meanscoupling an acoustical output of said transducer to said anisotropicmedium and wherein said means for varying the frequency of theacoustical waves includes a source of time varying electrical signalscoupled to said transducer to drive same, said source including meansfor varying the frequency content of the output electrical signalsdelivered to said transducer.

30. An optical filter as in claim 28 wherein said anisotropic medium isa crystal of lithium niobate.

31. Optical filters as in claim 30 wherein said crystal of lithiumniobate is oriented to propagate the input broad band light beam as anextraordinary wave along the axis of said crystal, the diffracted wavebeing an ordinary narrower band wave along the axis, the non entumvectors of said waves satisfying the condition F, k,, k,, where k, isthe momentum vector of said narrower band of the extraordinary wave, E,is the momentum vector of he acoustic wave and k, is the momentum vectorof the narrower band ordinary wave.

32. An optical filter as in claim 30 wherein said crystal is providedwith a face inclined at an angle to said input light beam and againstwhich the acoustic waves are directed to reflect from the inside of saidinclined face to travel collinearly with said input light beam.

33. An optical filter as in claim 32 wherein said acoustical waves areinitially longitudinal, and are converted into shear waves by reflectionfrom said surface.

34. An optical filter as in claim 28 in which said narrower band outputlight beam and acoustic wave propagate collinearly through saidanisotropic medium.

35. Optical filter as in claim 28 wherein said anisotropic medium is abirefringent crystal.

36. An optical filter as in claim 28 wherein said medium is abirefringent crystal having an input inclined face and a reflectivesurface at opposite ends.

37. An optical filter as in claim 36 in which said means for developingacoustical waves is coupled to said reflective surface.

38. The apparatus of claim 28 wherein said means for developing saidinput light beam comprises means for developing an incoherent inputlight beam.

39. An optical filter comprising, a light beam having a first band offrequencies polarized in a first polarization orientation direction, anisotropic medium disposed to receive said beam, said medium beingselected and arranged to propagate said light beam as particular waveshaving said first polarization orientation in a birefringent system,said medium being further capable of diffracting and propagating a lightbeam with a second band of frequencies narrower than and within saidfirst band and polarized in a second polarization orientation differentfrom said first beam as another wave of said birefringent system inresponse to acoustical waves, means coupled to said medium fordeveloping an acoustical wave of predetermined frequency in saidanisotropic medium, the frequency of said acoustical wave being selectedto define a matched relationship such that the vector sum of themomentum vectors of the narrower second band of frequencies within andof said first band of frequencies of the incident light beam and theacoustical wave are equal to that of the narrower frequency band oflight oriented at said second polarization, whereby said narrower secondband of said incident light beam satisfying said matched relationshipfor a given acoustical wave frequency is diffracted into said secondpolarization orientation, an output analyzer aligned to analyze thelight waves of said second polarization, and means for varying thefrequency of the acoustical wave to vary the frequency of light wavesbeing diffracted into said second polarization orientation.

40. An optical filter as in claim 39 in which said first polarizationorientation is linear in a predetermined direction, and in which saidsecond polarization orientation is linear in a direction orthogonal tothe first.

41. The apparatus of claim 39 wherein said light beam of first bandwidthand first polarization is a beam of incoherent light.

42. In an acousto-optic method the steps of, exciting an acoustic wavein an optically birefringent medium selected and arranged to propagatelight of a first polarization and of a first bandwidth as particularwaves in the medium cumulatively and substantially collinearlydiffracting said light of said first polarization and first bandwidth onthe acoustic wave within the optically birefringent medium toselectively cumulatively diffract only a narrower instantaneousbandwidth portion of said first band of frequencies of said light ofsaid first polarization into light of said second polarization.

43. The method of claim 42 including the step of analyzing thediffracted light beam to separate light of one of said polarizationsfrom light of the other of said polarizations.

44. The method of claim 42 including the steps of varying the frequencyof the acoustic wave in the optically birefringent medium to vary thecenter frequency of the narrower band of optical frequenciescumulatively diffracted to the second polarization within the widerbandwidth of light of the first polarization.

45. The method of claim 42 wherein the step of collinearly diffractingthe light beam on the acoustic wave includes the step of directing thelight beam to the difi'racted through the optically birefringent mediumsuch that the light beam to be diffracted passes into the birefringentmedium on one side thereof and the diffracted light beam emerges fromthe medium at a side opposite to the first side.

46. The method as in claim 45 including the steps of analyzing the lightbeam which emerges from the medium at the side opposite the first sideto separate light of one of said polarizations from light of the otherof said polarizations.

47. The method of claim 42 wherein said light of first polarizationwhich is selectively cumulatively collinearly diffracted on the acousticwave into light of said second polarization is incoherent light.

48. In an acousto-optic method for selectively diffracting only aportion of the bandwidth of light of a first polarization into light ofa second polarization the steps of, exciting an acoustic wave in anoptically birefringent medium selected and arranged to propagate lightas a particular wave in the birefringent medium, such acoustic wavebeing of an acoustic frequency related to the optical frequency of theportion of the bandwidth of the light to be diffracted from the first tothe second polarization, and cumulatively and substantially collinearlydiffracting a beam of light having a first optical bandwidth and thefirst polarization on the acoustic wave within the opticallybirefringent medium to selectively cumulatively diffract light of thefirst polarization and substantially only within an instantaneousoptical passband less than said first bandwidth into light of saidsecond polarization.

49. The method of claim 48 including the step of analyzing thediffracted light beam to separate light of one of said polarizationsfrom light of the other of said polarizations.

50. The method of claim 48, including the step of varying the frequencyof the acoustic wave in the optically birefringent medium to vary thecenter frequency of the instantaneous passband of optical frequenciescumulativelydiffracted to the second polarization within said widerfirst optical bandwidth of light to be diffracted.

51. The method of claim 48 including the step of linearly polarizing thelight beam to be diffracted in the direction of the first polarization.

52. The method of claim 48 wherein the step of collinearly diffractingthe light beam on the acoustic wave includes the step of, directing thelight beam to be diffracted through the optically birefringent mediumsuch that the light beam to be diffracted passes into the birefringentmedium on one'side thereof and the diffracted light beam emerges fromthe medium at a side opposite the first side.

53. The method of claim 48 including the step of directing the excitedacoustic wave relative to the medium at such an angle as to be deflectedat a boundary of the medium from a path which is inclined to the opticalbeam path into a path which is generally collinear with the optical beampath within the optically birefringent medium.

54. The method of claim 48 wherein the light of first polarization whichis diffracted into the second polarization is incoherent light.

i 4 t l l

1. In an acousto-optic apparatus for diffracting light of a firstpolarization and first bandwidth into light of a second polarizationincluding, optically birefringent medium means disposed to receive abeam of light having a first optical bandwidth and to be diffracted froma first polarization into a second polarization, means for exciting anacoustic wave in said optically birefringent medium at a frequencyrelated to the optical frequency of the light to be diffracted from thefirst to the second polarization, such acoustic wave having a path ofpropagation within said optically birefringent medium which issubstantially collinear with the optical beam path for cumulativelycollinearly diffracting the light beam on the acoustic wave within theoptically birefringent medium to selectively diffract light of the firstpolarization, and within a second instantaneous passband of opticalfrequencies narrower than said first bandwidth and related to thefrequency of the acoustic wave, into light of the second polarization,and means for developing said light beam to be diffracted with saidfirst optical bandwidth wider tHan said second narrower instantaneouspassband.
 2. The apparatus of claim 1 including, means for analyzing thediffracted light beam to separate light of one of said polarizationsfrom the light of the other of said polarizations.
 3. The apparatus ofclaim 1 including, means for varying the frequency of the acoustic wavewithin said optically birefringent medium over a band of acousticfrequencies to tune the second instantaneous passband of opticalfrequencies over an optical bandwidth.
 4. The apparatus of claim 1including, means for linearly polarizing the light beam to be diffractedin the direction of the first polarization.
 5. The apparatus of claim 1wherein said optically birefringent medium means is disposed to receivethe beam of light to be diffracted with a light beam path through saidmedium such that the light beam enters said medium at one side,traverses a beam path within said medium, and emerges from said mediumas a diffracted light beam on a side of said medium opposite the side oflight beam entry.
 6. The apparatus of claim 1 wherein said means forexciting an acoustic wave in said optically birefringent mediumincludes, acoustic transducer means coupled to said opticallybirefringent medium means for directing an excited acoustic wave intothe medium at such an angle as to be deflected at a boundary of saidoptically birefringent medium from a path which is inclined to theoptical beam path into a path which is generally collinear with theoptical beam path within said optically birefringent medium.
 7. Theapparatus of claim 1 wherein said means for developing said light beamof first polarization and first bandwidth develops said light beam as abeam of incoherent light.
 8. In a acousto-optic apparatus fordiffracting light of a first polarization into light of a secondpolarization including, optically birefringent medium means disposed toreceive a beam of light to be diffracted from a first polarization intoa second polarization such beam of light having a beam path through saidmedium such that the light beam enters the medium at one side, traversesa beam path within the medium, and emerges from the medium as adiffracted light beam on a side of the medium opposite to the side oflight beam entry, means for exciting an acoustic wave in said opticallybirefringent medium at a frequency related to the optical frequency ofthe light to be diffracted from the first to the second polarization,such acoustic wave having a path of propagation within said opticallybirefringent medium which is substantially collinear with a substantialcomponent of the optical beam path for cumulatively collinearlydiffracting the light beam on the acoustic wave within the opticallybirefringent medium to diffract light of the first polarization and ofan optical frequency related to the frequency of the acoustic wave intolight of the second polarization.
 9. The apparatus of claim 8 including,means for analyzing the diffracted light beam to separate light of oneof said polarizations from light of the other of said polarizations. 10.The apparatus of claim 8 including means for varying the frequency ofthe acoustic wave in said optically birefringent medium to vary theoptical frequency of the light diffracted from the first polarizationinto the second polarization.
 11. The apparatus of claim 8 including,means for developing said light beam to be diffracted with an opticalbandwidth wider than the instantaneous passband of the light which isdiffracted from the first polarization to the second polarization. 12.The apparatus of claim 8 including, acoustic transducer means forexciting and directing the acoustic wave into said opticallybirefringent medium at such an angle as to be deflected at a boundary ofsaid optically birefringent medium from a path which is inclined to theoptical beam path into a path which is generally collinear with theoptical beam path within said optically birefringent medium.
 13. Theapparatus of claim 8 wherein said birefringent medium means is abirefringent crystal.
 14. The apparatus of claim 29 including, means forlinearly polarizing the incident light beam in the direction of thefirst polarization.
 15. In an acousto-optic apparatus, a light beam of afirst polarization and bandwidth which is to have only a portion of itstotal handwith diffracted into light of a second polarization, opticallybirefringent medium means disposed to receive the beam of light having afirst optical bandwidth and to be diffracted from a first polarizationinto a second polarization, means for exciting an acoustic wave in saidoptically birefringent medium at a frequency related to the opticalfrequency of that portion of the bandwidth of the light to be diffractedfrom the first to the second polarization, such acoustic wave having apath of propagation within said optically birefringent medium which issubstantially collinear with a substantial component of the optical beampath within said birefringent medium for cumulatively collinearlydiffracting the light beam on the acoustic wave within the opticallyanisotropic medium to diffract light of the first polarization, andwithin a second instantaneous passband of optical frequencies narrowerthan said first bandwidth of light and related to the frequency of theacoustic wave, into light of the second polarization, means for varyingthe frequency of the acoustic wave within said optically birefringentmedium over a band of acoustic frequencies having a bandwidth in excessof 10 percent to tune the second instantaneous passband of opticalfrequencies for light diffracted from the first polarization to thesecond polarization over a comparable optical bandwidth in excess of 10percent.
 16. The apparatus of claim 15 wherein said means for varyingthe frequency of the acoustic wave includes an acoustic transducer meanshaving a bandwidth in excess of 10 percent for exciting acoustic wavesover a relatively wide band of acoustic frequencies.
 17. The apparatusof claim 15 wherein said means for varying the frequency of the acousticwave over a band of acoustic frequencies includes, tunable radiofrequency generator means for generating an electrical signal tunableover a band of frequencies as wide as the band of acoustic frequenciesto be generated within said optically birefringent medium, and acoustictransducer means responsive to the tunable electric signal for excitingthe acoustic waves to be excited in said birefringent medium means. 18.The apparatus of claim 15 including, means for developing said lightbeam to be diffracted with said first optical bandwidth wider than saidsecond instantaneous passband of light which is diffracted from thefirst polarization to the second polarization.
 19. The apparatus ofclaim 15 including, means for analyzing the diffracted light beam toseparate light of one of said polarizations from light of other of thesaid polarizations.
 20. The apparatus of claim 15 wherein said lightbeam of first polarization is a beam of incoherent light.
 21. Anacousto-optic apparatus comprising, means for developing a first lightbeam having a first band of frequencies with a predeterminedpolarization, an anisotropic crystal disposed to receive said beam, saidcrystal being selected and arranged to propogate said light beam as aparticular wave in a birefringent system, said crystal being furthercapable of diffracting and propagating a second light beam with a secondband of frequencies within said first band and being of orthogonalpolarization to said first beam, as another wave of said birefringentsystem in response to acoustical waves in the medium, means coupled tosaid crystal for developing acoustical waves of predetermined frequencyin said anisotropic crystal, the frequency of said acoustical wave beingselected to define a phase-matched relationship with said first lightbeam such that the vector sum of the momentum vectors of said secondband of frequencies narRower than and within said first band offrequencies of the incident light beam and the acoustical wave are equalto that of said second light beam orthogonally oriented to said inputlight beam, whereby said second narrower band of frequencies of saidincident light beam satisfying said phase-matched relationship for agiven acoustical wave frequency is diffracted into said orthogonalpolarization, and an output analyzer aligned to selectively pass thelight waves orthogonally related to the polarization of said input lightbeam.
 22. An acousto-optic apparatus as in claim 21 in which saidcrystal is provided with end faces forming an acoustic resonator, areflective surface carried on one of said end faces for internallyreflecting light waves from within said crystal.
 23. An acousto-opticapparatus as in claim 21 in which said crystal has a pair of end faces,one of said end faces being constructed and arranged to admit said firstlight beam into said crystal, light reflective means being provided atsaid other end face for internally reflecting light impinging thereon.24. An acousto-optic apparatus as in claim 22 wherein said means fordeveloping acoustical waves is coupled to said end face having saidlight reflective means.
 25. The apparatus of claim 21 wherein said meansfor developing said first light beam develops a first light beam as abeam of incoherent light.
 26. An acousto-optic apparatus including afirst light beam having a first band of frequencies and predeterminedpolarization, an anisotropic crystal medium disposed to receive saidfirst light beam, said crystal being selected and arranged to propagatesaid first light beam as a particular wave in a birefringent system,said crystal being further capable of diffracting and propagating asecond light beam having a second band of frequencies within andnarrower than said first band of frequencies at orthogonal polarizationto said first beam as another wave of said birefringent system inresponse to acoustical waves, means coupled to said medium fordeveloping acoustical waves of predetermined frequency in saidanisotropic medium, the frequency of said acoustical waves beingselected to define a phase-matched relationship such that the vector sumof the momentum vectors of said second narrower band of frequencies ofsaid incident light beam and the acoustical wave are equal to that ofsaid second band of frequencies of said second light beam orthogonallyoriented to said first input light beam, whereby said second band offrequencies of said incident light beam satisfying said phase-matchedrelationship for the given acoustical wave frequencies are diffractedinto said orthogonal polarization.
 27. The apparatus of claim 26 whereinsaid first light beam is a beam of incoherent light.
 28. An opticalfilter for deriving a narrower band of frequencies from a light beamhaving a first band of frequencies comprising, means for developing aninput light beam having a first band of optical frequencies and having apredetermined first polarization, an optically anisotropic mediumdisposed to receive said light beam, said medium being selected andarranged to propagate said light beam as particular light waves in abirefringent system, said medium being further capable of diffractingand propogating a second band of optical frequencies narrower than andwithin said first band of optical frequencies, of orthogonalpolarization to said first band as another wave of said birefringentsystem, in response to an acoustical wave in said medium, means coupledto said medium for developing acoustical waves of predeterminedfrequency in said anisotropic medium, the predetermined frequency ofsaid acoustical waves being selected to define a phase-matchedrelationship such that the vector sum of certain momentum vectors of theincident light beam and the acoustical wave are substantially equal tothe momentum vector of said second narrower band of light waves, wherebysaid second narrower band of optical freqUencies of the incident lightbeam satisfying said momentum vector condition for a given acousticalwave frequency is diffracted into said orthogonal polarization toprovide said second narrower band of optical frequencies, an outputanalyzer aligned to analyze the light waves orthogonally related to thefirst polarization of said input light beam, and means for varying thefrequency of the acoustical waves to vary the frequency of said secondnarrower band of the light waves being diffracted into said orthogonalpolarization.
 29. An optical filter as in claim 28 wherein said meansfor developing an acoustical wave includes an electro-acousticaltransducer, means coupling an acoustical output of said transducer tosaid anisotropic medium and wherein said means for varying the frequencyof the acoustical waves includes a source of time varying electricalsignals coupled to said transducer to drive same, said source includingmeans for varying the frequency content of the output electrical signalsdelivered to said transducer.
 30. An optical filter as in claim 28wherein said anisotropic medium is a crystal of lithium niobate. 31.Optical filters as in claim 30 wherein said crystal of lithium niobateis oriented to propagate the input broad band light beam as anextraordinary wave along the axis of said crystal, the diffracted wavebeing an ordinary narrower band wave along the axis, the momentumvectors of said waves satisfying the condition ke + ka ko, where ke isthe momentum vector of said narrower band of the extraordinary wave, kais the momentum vector of he acoustic wave and ko is the momentum vectorof the narrower band ordinary wave.
 32. An optical filter as in claim 30wherein said crystal is provided with a face inclined at an angle tosaid input light beam and against which the acoustic waves are directedto reflect from the inside of said inclined face to travel collinearlywith said input light beam.
 33. An optical filter as in claim 32 whereinsaid acoustical waves are initially longitudinal, and are converted intoshear waves by reflection from said surface.
 34. An optical filter as inclaim 28 in which said narrower band output light beam and acoustic wavepropagate collinearly through said anisotropic medium.
 35. Opticalfilter as in claim 28 wherein said anisotropic medium is a birefringentcrystal.
 36. An optical filter as in claim 28 wherein said medium is abirefringent crystal having an input inclined face and a reflectivesurface at opposite ends.
 37. An optical filter as in claim 36 in whichsaid means for developing acoustical waves is coupled to said reflectivesurface.
 38. The apparatus of claim 28 wherein said means for developingsaid input light beam comprises means for developing an incoherent inputlight beam.
 39. An optical filter comprising, a light beam having afirst band of frequencies polarized in a first polarization orientationdirection, an isotropic medium disposed to receive said beam, saidmedium being selected and arranged to propagate said light beam asparticular waves having said first polarization orientation in abirefringent system, said medium being further capable of diffractingand propagating a light beam with a second band of frequencies narrowerthan and within said first band and polarized in a second polarizationorientation different from said first beam as another wave of saidbirefringent system in response to acoustical waves, means coupled tosaid medium for developing an acoustical wave of predetermined frequencyin said anisotropic medium, the frequency of said acoustical wave beingselected to define a matched relationship such that the vector sum ofthe momentum vectors of the narrower second band of frequencies withinand of said first band of frequencies of the incident light beam and theacoustical wave are equal to that of the narrower frequency band oflight oriented at said second polarization, wherEby said narrower secondband of said incident light beam satisfying said matched relationshipfor a given acoustical wave frequency is diffracted into said secondpolarization orientation, an output analyzer aligned to analyze thelight waves of said second polarization, and means for varying thefrequency of the acoustical wave to vary the frequency of light wavesbeing diffracted into said second polarization orientation.
 40. Anoptical filter as in claim 39 in which said first polarizationorientation is linear in a predetermined direction, and in which saidsecond polarization orientation is linear in a direction orthogonal tothe first.
 41. The apparatus of claim 39 wherein said light beam offirst bandwidth and first polarization is a beam of incoherent light.42. In an acousto-optic method the steps of, exciting an acoustic wavein an optically birefringent medium selected and arranged to propagatelight of a first polarization and of a first bandwidth as particularwaves in the medium cumulatively and substantially collinearlydiffracting said light of said first polarization and first bandwidth onthe acoustic wave within the optically birefringent medium toselectively cumulatively diffract only a narrower instantaneousbandwidth portion of said first band of frequencies of said light ofsaid first polarization into light of said second polarization.
 43. Themethod of claim 42 including the step of analyzing the diffracted lightbeam to separate light of one of said polarizations from light of theother of said polarizations.
 44. The method of claim 42 including thesteps of varying the frequency of the acoustic wave in the opticallybirefringent medium to vary the center frequency of the narrower band ofoptical frequencies cumulatively diffracted to the second polarizationwithin the wider bandwidth of light of the first polarization.
 45. Themethod of claim 42 wherein the step of collinearly diffracting the lightbeam on the acoustic wave includes the step of directing the light beamto the diffracted through the optically birefringent medium such thatthe light beam to be diffracted passes into the birefringent medium onone side thereof and the diffracted light beam emerges from the mediumat a side opposite to the first side.
 46. The method as in claim 45including the steps of analyzing the light beam which emerges from themedium at the side opposite the first side to separate light of one ofsaid polarizations from light of the other of said polarizations. 47.The method of claim 42 wherein said light of first polarization which isselectively cumulatively collinearly diffracted on the acoustic waveinto light of said second polarization is incoherent light.
 48. In anacousto-optic method for selectively diffracting only a portion of thebandwidth of light of a first polarization into light of a secondpolarization the steps of, exciting an acoustic wave in an opticallybirefringent medium selected and arranged to propagate light as aparticular wave in the birefringent medium, such acoustic wave being ofan acoustic frequency related to the optical frequency of the portion ofthe bandwidth of the light to be diffracted from the first to the secondpolarization, and cumulatively and substantially collinearly diffractinga beam of light having a first optical bandwidth and the firstpolarization on the acoustic wave within the optically birefringentmedium to selectively cumulatively diffract light of the firstpolarization and substantially only within an instantaneous opticalpassband less than said first bandwidth into light of said secondpolarization.
 49. The method of claim 48 including the step of analyzingthe diffracted light beam to separate light of one of said polarizationsfrom light of the other of said polarizations.
 50. The method of claim48, including the step of varying the frequency of the acoustic wave inthe optically birefringent medium to vary the center frequency of theinstantaneous passband of optical frequencies cumulatively diffracted tothe second polarization within said wider first optical bandwidth oflight to be diffracted.
 51. The method of claim 48 including the step oflinearly polarizing the light beam to be diffracted in the direction ofthe first polarization.
 52. The method of claim 48 wherein the step ofcollinearly diffracting the light beam on the acoustic wave includes thestep of, directing the light beam to be diffracted through the opticallybirefringent medium such that the light beam to be diffracted passesinto the birefringent medium on one side thereof and the diffractedlight beam emerges from the medium at a side opposite the first side.53. The method of claim 48 including the step of directing the excitedacoustic wave relative to the medium at such an angle as to be deflectedat a boundary of the medium from a path which is inclined to the opticalbeam path into a path which is generally collinear with the optical beampath within the optically birefringent medium.
 54. The method of claim48 wherein the light of first polarization which is diffracted into thesecond polarization is incoherent light.